How Atomic Clocks Work

Atomic clocks are based on an idea first hit on by Isidor Rabi in the 1930s called atomic beam magnetic resonance. An atomic beam fired over a superconductor containing vortices will experience a time-dependent magnetic field.

Some people think that the atomic clock works by measuring the decay of some isotope. This is not the case; rather it measures oscillations of an atom, just as a pendulum clock measures the swing of a pendulum. In the atomic clock, cesium atoms are placed in a copper tube that is surrounded by laser beams (four perpendicular and at right angles to each other and one above and one below). When they are all turned on the cesium atoms gather at the very center of the tube. Then all of the lasers except the one beneath the copper tube are turned off. The one beneath is turned off and on and the cesium atoms are moved up and down in a fountain-like action. To ensure precision, the clock is shielded from the Earth's magnetic field and the temperature is set to near absolute zero (-273 degrees Centigrade).

As the cesium atoms are thrown up by the laser beneath the tube, they go through a microwave-emitting cavity and then return via gravity through the same cavity. The microwaves are fired at a fluctuating rate around 9,192,631,770 Hertz - regulated by a vibrating crystal. The cesium atoms actually begin to glow or emit light. As the atoms change state, their change is measured. When the state changes are at their optimum, the number of Hertz (cycles per second) of the microwave is measured. This is used to adjust the crystal oscillator, which should now register true seconds.

This all occurs at two places at once, in Colorado and in Paris. The two clocks really only register the length of a second. An average is found between the two measurements. This second is used to synchronize 200 or so other less accurate clocks stationed at other locations around the world. The average of all of these clocks is called International Atomic Time. The clock is so accurate that with the degree of possible error assigned it would take 10 million years for the clock to be off by a second. The Bureau of Weights and Measures then distributes this information throughout the world.

So why do we need such accuracy in time keeping? Very simply, modern technology demands it. GPS devices are accurate to within a few feet because they receive time signals from various satellites and calculate using the difference between them. The internet uses highly accurate clocks to synchronize its various parts. Astronomers and physicists use accurate time pieces to measure distances in space. A space probe could not be sent to a planet without the use of atomic clocks. It's very location is calculated using accurate time pieces.